Photovoltaic solar cells have many applications. Solar cell systems may be connected to an electric utility grid or be used independently. Applications include water heating, residential electric power, electric power for buildings, generation of power for electric utilities, applications in space, military applications, electric power for automobiles, airplanes, etc., and low-power specialty applications. Solar cells may be used in rooftop systems, in sheets rolled out on large flat areas in the desert or elsewhere, on systems that track the motion of the sun to gain the maximum incident solar power, with or without lenses and/or curved reflectors to concentrate the sun's light on small cells, in folding arrays on satellites and spacecraft, on the surfaces of automobiles, aircraft and other objects and even embedded in fabric for clothing, tents, etc.
The primary function of a solar cell is to convert electromagnetic radiation, in particular solar radiation, into electrical energy. The energy delivered by solar radiation at the earth's surface primarily contains photons of energy hv in the range 0.7 eV up to 3.5 eV, mostly in the visible range, with hv related to the wavelength λ of the light by hv=1.24 eV/λ (μm). Although many photons of longer wavelength are incident at the earth's surface they carry little energy.
Most semiconductor devices, including semiconductor solar cells, are based on the p-n junction diode. In a semiconductor, the lowest conduction band and the highest valence band are separated by an energy gap, Eg. A semiconductor is transparent to electromagnetic radiation with photons of energy hv less than Eg. On the other hand, electromagnetic radiation with hv≧Eg is absorbed. When a photon is absorbed in a semiconductor, an electron is optically excited out of the valence band into the conduction band, leaving a hole (an absence of an electron in a state that normally is filled by an electron) behind. Optical absorption in semiconductors is characterized by the absorption coefficient. The optical process is known as electron-hole pair generation. Electron-hole pairs in semiconductors tend to recombine by releasing thermal energy (phonons) or electromagnetic radiation (photons) with the conservation of energy and momentum.
When incident photons with energy equal to or greater than the energy gap of the semiconductor p-n junction diode are absorbed, electron-hole pairs are generated. Electron-hole pairs generated by the incident photons with energy greater than the band gap are called hot carriers. These photo-generated hot electrons and holes, which reside in the energy band away from the energy band zone center, rapidly give away their excess energy (the energy difference between the total carrier energy and the energy gap) to the semiconductor crystal lattice causing crystal lattice vibrations (phonons), which produce an amount of heat equal to the excess energy of the carriers in the semiconductor. As a result of the photo-generated electrons and holes moving in opposite directions under an electric field within the semiconductor p-n junction diode, electron and hole photocurrents are simultaneously generated. Semiconductor devices based on this operating principle are known as photodiodes. Semiconductor photovoltaic solar cells are based on the same operating principle as the semiconductor p-n junction photodiodes described above.
A conventional single p-n junction photovoltaic solar cell is composed of a very thick (p) and a very thin (n) semiconductor or vice versa. The thick (p−) doped absorber layer on the bottom of the photovoltaic solar cell is called the base, while the thin (n) layer on the top of the photovoltaic solar cell is called the emitter. The ideal efficiency of a photovoltaic solar cell is the percentage of power converted from the absorbed electromagnetic radiation to electrical energy. The photovoltaic solar cell energy conversion efficiency is partially determined by the band gap of the base layer semiconductor.
The advantage of a photovoltaic solar cell with a small energy gap base layer is that more incident photons are absorbed, and hence more electron-hole pairs are generated, producing a relatively high current in the solar cell. One disadvantage of such a photovoltaic solar cell is that the photovoltage is relatively low due to the small energy gap of the absorber. Another disadvantage of a small energy gap photovoltaic solar cell is that hot carriers are generated by the incident photons with energy much greater than the energy gap, and hence the excess energy of the hot carriers produces a large amount of heat in the thermalization process unless the higher energy photons are absorbed before reaching the narrow-gap material.
On the other hand, the advantage of a photovoltaic solar cell with a large energy gap base layer is that the output voltage of the photovoltaic solar cell is relatively high due to the large energy gap of the absorber. In addition, fewer hot carriers are generated because there are fewer photons with energy much greater than the energy gap. The disadvantage of such a photovoltaic solar cell is that a large number of incident photons have energies below the energy gap of the base layer semiconductor and hence are not absorbed, so that the output current is relatively low.
To achieve high energy conversion efficiency for a semiconductor photovoltaic solar cell, a high output voltage and a high current are required. In order to take advantage of narrow and wide band gap photovoltaic materials, a multifunction photovoltaic solar cell architecture approach is employed by stacking a number of photovoltaic solar cells with various base layer energy gaps. By connecting the photovoltaic solar cells in a serial fashion with the base layer energy gaps spanning the entire solar spectrum, optimal energy conversion efficiency could be achieved. But, in practice, the base layer energy gaps of a multijunction solar cell only cover a portion of the entire solar spectrum. To obtain maximum energy conversion efficiency in a multijunction photovoltaic solar cell, each individual solar cell (p-n junction diode) must be fabricated with high electrical and optical quality semiconductors, which can be achieved for lattice matched single-crystal semiconductor systems grown by molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), liquid phase epitaxy (LPE), or other epitaxial growth techniques.
In addition, the photocurrent generated in each individual solar cell is optimally identical to that in the others in order to maximize the energy conversion efficiency since the individual solar cells are connected in series and the photocurrent flows through each individual solar cell in a serial fashion. Any excess current due to current mismatching among the individual solar cells is converted into heat in the multijunction photovoltaic solar cell. The photocurrent of a single p-n junction photovoltaic solar cell is proportional to the number of photons absorbed, which varies directly with the thickness and absorption coefficient of the base and emitter layers. Hence, the thicknesses of the semiconductor layers in a semiconductor multijunction photovoltaic solar cell must be properly designed based on the factors mentioned above in order to match the photocurrent generated in each individual solar cell.
Degenerately alloyed thin (p++) and (n++) tunnel junctions (TJ) are used as electrical circuit interconnects in a large number of multijunction photovoltaic solar cells to increase the solar energy conversion efficiency. The tunnel junctions are designed with minimal resistance and voltage drops across the junctions because the photovoltaic voltage of a multijunction solar cell is the sum of the photovoltaic voltage of the individual cells minus the voltage drops across the electrical circuit interconnects and contacts. A typical tunnel junction consists of an interface of heavily alloyed (p++) and (n++) layers with a narrow depletion layer in which a thin barrier is formed for electron tunneling. The separation of photo-generated electron-hole pairs due to the space-charge electric field induces a voltage drop across the tunnel diodes. In order to maximize the solar energy conversion efficiency, the voltage losses in the tunnel diodes must be minimized, and in addition the current of the individual photovoltaic cells must be matched. Under forward bias, electrons tunnel from the (n++) alloyed to (p++) alloyed layers, while electrons tunnel from (p++) alloyed to (n++) alloyed layers under reverse bias.
To further improve the solar energy conversion efficiency of the photovoltaic solar cells, three-junction device structures have been employed. To date, Group III-V three-junction photovoltaic solar cells have been the most successful solar cell device architectures in terms of solar energy conversion efficiencies. Examples of such Group III-V three-junction photovoltaic solar cells are InGaP/GaAs/Ge and InGaP/InGaAs/Ge photovoltaic solar cells that are grown on Ge substrates by MBE or MOCVD. These cells have a conversion efficiency of approximately 40%. Other photovoltaic solar cell device structures with similar solar energy conversion efficiencies are the InGaP/GaAs/InGaAs three-junction photovoltaic solar cells grown on GaAs substrates by MBE or MOCVD. To optimize the total output current, degenerately alloyed (p++)GaAs/(n++) GaAs, (p++) AlGaAs/(n++) InGaP, and (p++) AlGaAs/(n++) GaAs tunnel junctions are often used.
Multijunction photovoltaic solar cells with four, five, and six junctions grown on Ge substrates have been proposed to achieve solar energy conversion efficiency greater than 45%. Examples of such Group III-V cells include AlInGaP/AlInGaAs/InGaAs/Ge, AlInGaP/AlInGaAs/InGaAs/InGaNAs/Ge, and AlInGaP/InGaP/AlInGaAs/InGaAs/InGaNAs/Ge. The tunnel junctions used in these multijunction photovoltaic solar cells are similar to those used in the three-junction photovoltaic solar cells described above.
Important considerations for achieving high-efficiency energy conversion include the following: a) high quality crystalline layers; b) an appropriate choice of junction band gaps based on the impinging solar spectrum; c) tunnel junction interconnects between p-n junctions; d) an appropriate choice of layer thicknesses to achieve a current-matched structure; and e) passivating layers, such as back-surface-field layers or window layers, to reduce losses.
In the past, high-efficiency III-V semiconductor multi-junction solar cells have been grown on GaAs, InP, and Ge substrates, but silicon substrates are advantageous for reasons of cost and mechanical robustness. The current multijunction single-crystal III-V solar cells grown on Ge substrates cost approximately $13/cm2, compared with approximately 2¢/cm2 for crystalline Si solar cells. However, sunlight incident on a small solar cell can be multiplied by a factor of 600 in a concentrator photovoltaic (CPV) system that tracks the sun to an accuracy of better than 1°. Thus, the cell cost per watt of electric power produced by a multijunction cell in a CPV system can be less than that of a Si cell in a flat plate system.
Previous efforts on the development of multijunction single-crystal solar cells have focused almost entirely on III-V materials for two compelling reasons. First, according to the commonly accepted wisdom, epitaxial growth is best performed on lattice matched substrates and on lattice matched epilayers, and it is very difficult to lattice match substrate material for the growth of appropriate II-VI layers for solar cells, whereas the GaInP/GaAs/Ge system is almost perfectly lattice matched and has an almost ideal set of energy gaps for a three-junction solar cell. See U.S. Pat. Nos. 6,657,194 and 6,906,358, which are specifically incorporated by reference.
Second, III-V materials and their doping and contacting are very familiar to many workers because of their widespread use in the electronics industry, whereas II-VI materials have been used only on a much more limited basis. Some representative patents for III-V based solar cells for CPV systems and previous cells using GaAs substrates are U.S. Pat. Nos. 4,163,987, 4,191,593, 4,206,002, 4,332,974, 4,575,577, 4,667,059, 4,926,230, 5,009,719, 5,342,453, 5,405,453, 5,853,497, 6,147,296, 6,252,287, 6,281,426, 6,300,557, 6,300,558, 6,660,928, 6,951,819, and 7,217,882.
In general, even a completely successful growth of a three-junction single-crystal III-V solar cell on Si would not solve all of the problems associated with multifunction III-V solar cells. In particular, the growth of III-V materials by MOCVD using hydrogen, arsine, phosgene and the other necessary precursor gases introduces a number of difficulties. This method of growth requires elaborate safety precautions and makes regulatory approval difficult. Also, in this method deposits appear rapidly in the growth chamber, which combined with the nature of the deposits implies high maintenance costs and much down time. These considerations make the development of II-VI multifunction single-crystal cells grown by MBE, as proposed in the present invention, very desirable.
The number of relevant patents dealing with Group II-VI solar cells is very limited. U.S. Pat. No. 4,710,589 teaches a heterojunction p-i-n photovoltaic cell having at least three different semiconductor layers formed of at least four different elements comprising a (p−) relatively wide band gap semiconductor layer, a high resistivity intrinsic semiconductor layer, used as an absorber of light radiation, and an (n) relatively wide band gap semiconductor layer. In the preferred embodiment ZnTe is employed as the (p) layer, CdTe as the intrinsic absorber layer, and CdS as the (n) layer.
U.S. Pat. No. 4,753,684 proposes a cell having only a single polycrystalline absorber layer. The proposed cell structure includes a relatively wide optical bandgap energy window layer, a light-absorbing layer and a third, relatively wide bandgap energy layer that forms a minority carrier mirror with the light-absorbing layer. It is realized using II-VI semiconductor compounds such as a CdS or ZnS window layer, a HgCdTe, CdTe, ZnCdTe or HgZnTe light absorbing layer and a third layer of CdTe, ZnTe, ZnCdTe, HgZnTe or CdMnTe. Cd and Te are present in at least two of the three layers of the proposed structures.
U.S. Pat. No. 6,419,742 proposes a method for the growth of high quality lattice mismatched II-VI semiconductor epitaxial layers on Si. This third patent proposes the formation of a passivation layer on a Si surface before the MBE growth of a II-VI material such as CdS. The passivation layer may comprise arsenic, germanium or CaF2.
Thus, there exists a need for low cost, highly efficient solar cells to help meet the power needs of the future. If ultrahigh efficiency multifunction II-VI solar cells could be manufactured by MBE using Si substrates, their manufacture would be easier to scale up than the manufacture of the corresponding III-V cells and would be substantially less expensive than even the corresponding III-V cells grown on Si substrates.
The only public disclosure of significant relevance to this invention originated from the first inventor, Prof. S. Sivananthan. He contracted Prof. M. Flatte of the University of Iowa to perform calculations of the possible theoretical efficiency of II-VI HgCdZnTe solar cells. The idea was confined to single-junction and two-junction solar cells with an unspecified substrate which was not an active part of the solar cell. There was no thought of applications to CPV systems or to applications in space. No publications resulted, only a workshop talk on the calculations by one of Prof. Flatte's students: “HgCdZnTe Materials for High-Efficiency Tandem Solar Cells”, B. Brown, M. E. Flatté, P. Boieriu, and S. Sivananthan, the 1998 U.S. Workshop on the Physics and Chemistry of II-VI Materials, Charleston, S.C., Oct. 21, 1998. No printed publication of these calculations was made and there was insufficient information in the art at the time to reduce these cells to practice.